i have an idea on what some of the 20 free dlc might be

[dark__gamer]

o.k soory if the dlc have been released.

but http://www.ebgames.com/Catalog/ProductDetails.aspx?product_id=72695

that link is the rock band tracks pack volume 2.

most of the songs are already DLC

[mathieupm13]

All those songs are already avaible as DLC and the 20 free Dlc are supposed to be 20 new songs

[Ryuzaki]

That's just a compilation of Rock Band DLC that you can get for a discount if you buy it in bulk. So instead of buying 20 songs normally for $40, you can buy 20 particular songs for $30.
Either that, or it's an offering for the PS2, since I'm not sure if they get DLC or not.

The 20 DLC songs that will be released this Fall will be all-new, absolutely free, and actually a part of Rock Band 2's original setlist. They were, supposedly, meant to be included on the game disc as part of the original setlist, but there wasn't enough space. So, instead, they'll be releasing them as free DLC.

[psyniac_123]

As said above; these are for people who can't get DLC on their console of choice (i.e. PS2, Wii).